well, you'll either have to reskin the unit yourself in the nif and kfm files(off the redcoat), or make a request on the board and someone MAY do it for you.....
once you have the new units art done, you'll have to make a new tech, which comes between muskets and rifles in the tech tree.... the longer you want the units the last, (production wise) the more space between the techs, obviously.
then, if you want it available for ALL civs you'll have to find out how to add it to the list of units, i'm not sure how to do that one, I don't think it's in the XML. Unless you want to make it a unique unit for only one or 2 civs, then you can just create the class, and specify in the units location of those civs.
you'll probably need help from a pythoner on this step i'm guessing.
good luck